Modeling a car wheel arc: Move vertex to form an arc
There are two tools that can help you achieve that.

1) Scale to sphere (Alt + Shift + S)

2) LoopTools Addon to space out vertices.

I recommend you proceed like this:

Scale to sphere


Set the 3D cursor to the center of your arc. We will use it as the pivot point for scaling the vertices.
press "." (NOT on the NumPad) to set the pivot point to the 3D cursor
select all the vertices you want to sit on the arc.
press Alt + Shift + S and move your mouse until the vertices align on an arc.


Space the vertices


Enable the LoopTools Addon in the User Preferences
with the vertices selected, press W and select LoopTools - Space